Circuit configuration and method for controlling a traction slip control system with brake and/or engine management

ABSTRACT

A circuit configuration and method for a traction slip control system which evaluates the speed (v ER ) measured at a driven spare wheel with a correction factor K(t) in order to maintain or improve the control function even when a smaller size spare wheel has been mounted. This correction factor (K(t)) is determined by axlewise comparison of the rotating speeds (v na1 , v na2  ; v a , v ER ) of the wheels of one axle and by comparison of the speed differences measured on the driven and nondriven axles, with traction slip control being inactive. Upon transition from a very slippery road surface (μ low  homogeneous) to a dry road surface (μ high  homogeneous), without any prior determination of the correction factor, the slip threshold (S) is raised temporarily. When starting with different right/left friction coefficients (μ-split), with the spare wheel being mounted on the high friction coefficient side, a higher slip threshold (S ER ) will be effective for this spare wheel.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/EP91/02108 filed Nov. 7, 1991.

BACKGROUND OF THE INVENTION

This invention relates to a circuit configuration and method formaintaining or improving the control function for a traction slipcontrol system with brake and/or engine management for automotivevehicles when a smaller spare wheel has been mounted as a driven wheelinstead of a normal size wheel. In the present invention, the rotatingspeed of the driven wheels is compared with the vehicle speed or with acorresponding measurement parameter for the purpose of detecting thetraction slip. A correction factor is obtained from the rotating speedsmeasured with stable rotational behavior of the wheels. Traction slipcontrol is triggered when wheel slip exceeds a predetermined limitvalue, namely the so-called slip threshold.

There are known circuit configurations for traction slip controlsystems. The information needed for control is obtained by means ofwheel sensors, important information being developed from the comparisonof the rotational behavior of the individual wheels and bydistinguishing between the driven wheels and the non-driven wheels.

Replacing a normal wheel by an emergency wheel of the type in currentuse today, the dimensions of which often differ considerably from thoseof normal wheels, can lead to faulty information for traction slipcontrol.

What, above all, is critical is the replacement of a driven wheel by anemergency wheel, the diameter of which is smaller than the diameter ofthe normal wheel. The traction slip control system identifies the higherrotating speed of the emergency wheel, caused by the smaller diameter,as "traction slip", with the control system responding and meteringbraking pressure into the wheel brake of the emergency wheel. Thismetering-in of braking pressure may completely prevent the car fromstarting in some situations such as in case of different right/leftfriction coefficients when the emergency wheel is located on the higherfriction coefficient side. Up to now, in order to avoid this situation,it was necessary to turn off the traction slip control when an emergencywheel has been mounted.

SUMMARY OF THE INVENTION

It is thus an object of this invention to overcome these disadvantageslinked with using emergency wheels in vehicles with a traction slipcontrol system.

It has been found that this object can be achieved by a circuitconfiguration and method by which the correction factor is determined byaxle-wise comparison of the rotating speeds of the two wheels of oneaxle and by comparing the speed differences measured on the driven axleand on the non-driven axle. The rotating speed measured at the sparewheel is evaluated by means of the correction factor and is therebyadapted to the measured value of the rotating speed of the second drivenwheel of the same axle, so that the slip threshold, decisive fortraction slip control, will become the same for the spare wheel as for anormal wheel.

Control will be improved in this way when, during the start or in acertain phase, traction slip control is not immediately activated. Thusthe control system has sufficient time to determine the correctionfactor and effect a corresponding adaptation. If, however, traction slipcontrol is activated at once when, for example, starting on a veryslippery road surface, a variant of the inventive circuit configurationand method will become operative. In this arrangement of the invention,at first, the same slip threshold is decisive for the control of the twodriven wheels upon the onset traction slip control whereupon the slipthreshold is raised until pressure reduction comes about if, during atraction slip control operation, pressure reduction signals fail toappear after a predetermined period of time. If, during a period of timeof the magnitude ranging from 200 to 500 msec, only braking pressurereduction takes place, this will be an indication of the completion ofthe traction slip control operation. Thereupon, the slip threshold willbe lowered again stepwise or continuously to the normal slip threshold.At the same time, it will again be possible to determine a correctionfactor in the afore-described manner, which correction factor serves toevaluate the rotating speed of the faster smaller emergency wheel.

Finally, there is provided yet another variation of the inventivecircuit configuration and method when different left/right frictioncoefficients exist during a starting operation, one wheel becomingunstable and the smaller spare wheel being located on the higherfriction coefficient side. For the purpose of maintaining or improvingthe control function, according to this arrangement of the invention,the slip threshold, decisive for the spare wheel, is raised up to theamount of deviation of the rotating speed of this spare wheel from therotating speed of the non-driven normal wheels. The rise takes placewith a predetermined gradient limited to approximately 0.6 . . . 0.8 g.

Further characteristics, advantages and applications of the presentinvention will become evident from the following description, referencebeing made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the speed variation, as a function of time, of the vehiclewheels during a starting operation, with traction slip control not beingactivated;

FIG. 2 shows the speed variation, as a function of time, of the vehiclewheels during a traction slip control operation and with a transitionfrom low friction coefficients on both sides to high frictioncoefficients on both sides;

FIG. 3 shows the speed variation, as a function of time, of the vehiclewheels during a starting operation and with different left/rightfriction coefficients (μ-split); and

FIG. 4 is a block diagram of a circuit configuration constructed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

All the FIGS. 1, 2 and 3 are based on measurements taken in anautomotive vehicle with a driven axle and a non-driven axle. Each wheelis equipped with an individual wheel sensor. In all cases, the controlsystem is a traction slip control system with brake management which canbe expanded by an engine management system if required. In all cases, aspare wheel is mounted instead of a driven normal wheel. The diameter ofthe spare wheel is only about 80% of the normal wheel diameter. In suchcases, i.e., when a considerably smaller spare wheel is mounted on thedriven axle, the result has had such grave implications for a tractionslip control system that, up to now, the control had to be put out ofoperation.

FIG. 1, on the one hand, shows the speed of a non-driven wheel v_(na1)or v_(na) 2. Upon starting, as slip is practically nil, this speedcorresponds to the vehicle speed. On the other hand, there are shown thespeed of the driven normal wheel v_(a) and the speed of the spare wheelv_(ER). Finally, the evaluated or, rather, the corrected speed of thespare wheel K(t)·v_(ER) is shown in FIG. 1. Besides these wheel speeds,FIG. 1 shows the variation of the correction factor K(t) during such astarting operation, with traction slip control not being activated.

In FIG. 1, a dash-dot line indicates the variation of the so-called"slip threshold" S which is defined by the spacing between the dash-dotline S and the vehicle speed represented by the speed of a non-drivenwheel v_(na). For the sake of facilitating the start, in the presentexample, the slip threshold S begins at a relatively high slip valuesuch as of 6 km/h, thereupon continuously decreasing to approximately 3km/h. In this example, this value will be reached at time t₃ when thespeed v_(na) of the vehicle will have gone up to about 20 km/h.Subsequently, the slip threshold will stay constant until time t₄, whenthe vehicle speed will have reached 60 km/h, and thereafter increasesfurther. The optimal variation of the slip threshold will depend on therespective vehicle type.

The process of adaptation or, rather, the correction of the speedv_(ER), measured at the spare wheel, will start at time [t] t₀ and willpractically have been completed at time t₁ which, in the presentexample, will be after 1.4 seconds. The precondition of this "learningprocess" is that the traction slip control system will remaininactivated for a sufficient period of time during the startingoperation. By means of axle-wise comparison of the speeds at the drivenand non-driven wheels in accordance with the relationship:

    Δ=(K×v.sub.ER -v.sub.a)-B(v.sub.na1 -v.sub.na2)

and by comparing the differences measured on the two axles, thecorrection factor K or, rather, K(t) is "learned" during the startingoperation when there is no excessive slip putting traction slip controlinto operation. "B" is a constant whose value is "1" in this example.Generally, the value of "B" lies between 0.3 and 1.0. If the result ofthis formation of differences in accordance with relationship (1)respectively exceeds zero or equals zero, K(t₁) will be corrected by thefactor k₁ after period of time T₁. In other words, if:

Δ≧0, then K(t)=K(t-T₁)-k₁.

Correspondingly, if:

Δ<0, then K(t)=K(t-T₂)+k₂ will apply.

Expediently, identical correction intervals T=T₁ =T₂ and identicalcorrection constants k=k₁ =k₂ are chosen. In one example, the correctioninterval T was chosen to be a period of time of 50 to 100 msec and thecorrection value k was selected to be a value ranging between k=0.005and k=0.01.

From FIG. 1, it can be seen that, without any correction of the speedv_(ER), the spare wheel would reach the slip threshold S at time t₂.Thus, the control logic would erroneously signal the existence ofexcessive traction slip at time t₂ and meter braking pressure into thewheel brake of the respective wheel so as to reduce this apparenttraction slip. According to this invention, such an undesired reactionof the traction slip control system will be prevented by a circuitconfiguration and method which determines a correction factor K(t) anduses the same to evaluate the rotating speed, measured at the sparewheel, in accordance with the above relationship (1). As of time t₁, atthe latest, the excessive speed of the smaller spare wheel or emergencywheel will have been compensated to such an extent as to ensure that theslip threshold S will be suited without any alteration also forevaluating the rotational behavior of this spare wheel so that, at alater time, traction slip control at the spare wheel will, indeed, notcome about before the desired time or only when a predetermined tractionslip value will have been exceeded.

FIG. 2 illustrates a situation in which, before activating traction slipcontrol, there is not sufficient time available for "learning" thecorrection factor K(t) or rather for adapting the circuitry to the sparewheel in the manner described with reference to FIG. 1. The slipthreshold S (i.e. the spacing between the line S and the vehicle speedrepresented by the speed of a non-driven wheel v_(na)) may, forinstance, be reached and exceeded very fast when starting on an icyroad--μ_(low) homogeneous. This is indicated by the zone on the left ofFIG. 2. The speed variation v_(a) of the driven normal wheel, as well asthe speed variation v_(ER) of the spare wheel, show a typical controlbehavior, the diameter of the spare wheel of this example also being 80%of the normal wheel diameter.

The troublesome effect of control by the smaller diameter of the sparewheel will be felt, in the present example, after time t₆. After timet₆, the vehicle or, rather, the driven wheels will enter a high frictioncoefficient zone marked μ_(high) homogeneous in FIG. 2. The speed v_(a)of the driven normal wheel will go below the slip threshold at time t₆and will subsequently remain in the stable zone. Speed v_(a) approachesspeed v_(na) of the non-driven wheels. Because of the smaller diameter,the uncorrected speed v_(ER) of the spare wheel, however, will stabilizeon a value above the slip threshold S determined for the normal wheelsize. As a consequence, again, there would be an undesired metering-inof braking pressure. This condition will be recognized by the inventivecircuit configuration and method because of the failure of brakingpressure reduction signals to appear at the spare wheel. This situationwill prevail after time period T_(A), i.e. after time t₇. Time periodT_(A), for example, will be approximately 1 sec. Consequently, as oftime t₇, the control unit will raise the slip threshold S until brakingpressure reduction signals will appear at the spare wheel. This will bethe case at time t₈ as shown in FIG. 2. After time t₈, the slipthreshold S', at first, will remain constant on the higher level andwill again be lowered if no renewed pressure build-up will have becomenecessary after the expiration of a certain period of time T_(E), suchas 200 to 400 msec, after the first onset of the pressure reductionpulses. After this period of time T_(E) or, rather, at time t₉, thecontrol unit will have determined that traction slip control will beover and will initiate a relatively slow downgrading of the slipthreshold S' to the original normal value S. Now follows a phase ofinactivated traction slip control and the "learning process" will set inas described with reference to FIG. 1. The broken characteristic linev_(ER) represents this wheel variation. In this learning phase, asdescribed, the speed v_(ER) of the spare wheel will be lowered by thecorrection factor K(t) so that the normal slip threshold S will becomedecisive for the spare wheel, too.

FIG. 3 relates to another situation in which, without the aid of theinventive circuit configuration and method, a smaller spare wheel woulddisturb the traction slip control system or put it out of operation. Inthis case, the friction coefficients at the two driven wheels of oneaxle are very different during the starting operation, i.e. the μ-splitsituation. The smaller diameter spare wheel is mounted on the higherfriction coefficient side. FIG. 3 shows the speed v_(na) of thenon-driven wheel which represents the vehicle speed, the speed v_(a) ofthe driven normal wheel which is influenced by traction slip control,the speed v_(ER) of the stable-running spare wheel and the slipthreshold S.

At time t₁₀, the speed v_(ER) of the spare wheel will exceed the slipthreshold S. This would result in a metering-in of braking pressure intothe wheel brake of the stable spare wheel running on a high frictioncoefficient. In such a case, however, according to the presentinvention, the normal slip threshold S will be raised to the slipthreshold S_(ER) for the spare wheel running on the higher frictioncoefficient. The difference between the normal slip threshold S and theslip threshold S_(ER) of the spare wheel will compensate for thedifference in the rotating speeds of the normal wheel and of the smallerspare wheel so that, in this situation, braking pressure will beprevented from being metered into the wheel brake of the spare wheel andtraction slip control may be performed only on the wheel with speedv_(a) running on a low friction coefficient.

After the completion of traction slip control, in this case, there willalso come about a correction by means of the circuit configuration andmethod described with reference to FIG. 1, whereupon, again, the sameslip threshold may apply to the two driven wheels.

The circuit configuration illustrated in FIG. 4, which represents oneform of an apparatus for carrying out the control method of the presentinvention just described, includes means for developing speed rotationsignals representative of the rotational speeds of the wheels of anautomotive vehicle. Such means are represented by blocks 10, 12, 14 and16, each of which can include a sensor, of conventional construction andoperation, which senses the rotational behavior (i.e. speed) of anassociated wheel and processing circuitry which produces a signalrepresentative of the rotational behavior of the associated wheel. Block18 represents the speed sensor and processing circuitry associated witha spare wheel of smaller diameter than normal mounted on the driven axleof the automotive vehicle. Block 16 represents the speed sensor andprocessing circuitry associated with the other wheel mounted on thedriven axle. Blocks 12 and 14 represent the speed sensor and processingcircuitry associated with the two wheels mounted on the non-driven axle.

The circuit configuration illustrated in FIG. 4 also includes means fordeveloping a correction factor, prior to traction slip control from thespeed rotation signals. Such means include, for the embodiment of theinvention illustrated in FIG. 4, a first comparator 20 to which thespeed rotation signals associated with the wheels on the non-driven axleare supplied and a second comparator 22 to which the speed rotationsignals associated with the wheels of the driven axle are supplied.Comparator 20 develops a first difference signal representative of thedifference in rotational speeds of the wheels mounted on the non-drivenaxle and comparator 22 develops a second difference signalrepresentative of the difference in the rotational speeds of the wheelsmounted on the driven axle. The difference signals developed bycomparators 20 and 22 are supplied to a third comparator 24 whichdevelops a correction factor signal representative of the differencebetween the difference in rotational speeds of the wheels mounted on thenon-driven axle and the difference in the rotational speeds of thewheels mounted on the driven axle.

The circuit configuration illustrated in FIG. 4 further includes meansfor adapting the speed rotation signal representative of the rotationalspeed of the spare wheel by the correction factor signal to therotational speed of the other wheel mounted on the driven axle. This isindicated by the connection from the output of the comparator 24 toblock 18 at which the speed rotation signal, representative of therotational speed of the spare wheel, is developed. The dashed lineextending through block 18 to block 16 represents the mounting of thesmaller than normal spare wheel on the driven axle to replace the otherwheel mounted on the driven axle.

A first processing circuit 26 develops a vehicle speed signal,representative of the speed of the automotive vehicle, from the speedrotation signals in the usual manner. A second processing circuit 28determines the onset of traction slip by comparing the rotational speedsof the wheels, represented by the speed rotation signals, including theadapted speed rotation signal of the spare wheel, with the vehiclespeed, represented by the vehicle speed signal developed by processingcircuit 26.

The circuit configuration illustrated in FIG. 4 finally includes meansfor establishing a slip threshold which must be exceeded before tractionslip control can commence and means for comparing the output ofprocessing circuit 28, representative of the onset of traction slip,with the slip threshold. Establishing the slip threshold is representedby a block 30 which can be any suitable means for setting a desired slipthreshold in a comparator 32 to which the output from processing circuit28 is supplied. When the output from processing circuit 28 exceeds theslip threshold, the output from comparator 32 permits slip controloperation to commence.

We claim:
 1. For an automotive vehicle having an undriven axle with two wheels mounted thereon and a driven axle with two wheels mounted thereon, one of said wheels mounted on said driven axle being a spare wheel which is smaller in diameter than a normal wheel, a method for controlling traction slip of said automotive vehicle comprising the steps of:measuring the rotating speeds of said wheels; developing a correction factor by:(a) comparing the measurements of said rotating speeds of said wheels mounted on said undriven axle to determine the difference in said rotating speeds of said wheels mounted on said undriven axle, (b) comparing the measurements of said rotating speeds of said wheels mounted on said driven axle to determine the difference in said rotating speeds of said wheels mounted on said driven axle, and (c) comparing said difference in said rotating speeds of said wheels mounted on said undriven axle with said difference in said rotating speeds of said wheels mounted on said driven axle; adapting said measurement of said rotating speed of said spare wheel by said correction factor to said measurement of said rotating speed of the other wheel mounted on said driven axle; establishing a slip threshold representative of a traction slip level which must be exceeded before traction slip control can commence; developing a measurement related to the speed of said automotive vehicle; determining the onset of traction slip by comparing said measurements of said rotating speeds of said wheels, including the adapted measurement of said rotating speed of said spare wheel, with said measurement related to the speed of said automotive vehicle; and initiating traction slip control by comparing the determination of the onset of traction slip with said slip threshold and determining when the determination of the onset of traction slip exceeds said slip threshold.
 2. A method for controlling traction slip according to claim 1 wherein said correction factor is developed during start-up of the vehicle prior to traction slip control.
 3. A method for controlling traction slip according to claim 2 wherein said correction factor is developed by stepwise adaptation.
 4. A method for controlling traction slip according to claim 2 wherein said correction factor is developed by reduction of said differences with said correction factor included.
 5. A method for controlling traction slip according to claim 2 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor reduced by a specific amount at predetermined time intervals of Δ is zero.
 6. A method for controlling traction slip according to claim 5 wherein said correction factor is reduced by identical amounts if the predetermined time intervals are identical.
 7. A method for controlling traction slip according to claim 6 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 8. A method for controlling traction slip according to claim 2 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor reduced by a specific amount at predetermined time intervals if Δ is greater than zero.
 9. A method for controlling traction slip according to claim 8 wherein said correction factor is reduced by identical amounts if the predetermined time intervals are identical.
 10. A method for controlling traction slip according to claim 9 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 11. A method for controlling traction slip according to claim 2 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)×f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor raised by a specific amount at predetermined time intervals if Δ is greater than zero.
 12. A method for controlling traction slip according to claim 11 wherein said correction factor is raised by identical amounts if the predetermined time intervals are identical.
 13. A method for controlling traction slip according to claim 12 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 14. A method for controlling traction slip according to claim 13 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-B(v.sub.na1 -v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheel B is a constant within the range of 0.3 to 1.0.
 15. A method for controlling traction slip according to claim 1 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-B(v.sub.na1 -v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheel B is a constant within the range of 0.3 to 1.0.
 16. A method for controlling traction slip according to claim 1 wherein said correction factor is developed during normal driving of the vehicle prior to traction slip control.
 17. A method for controlling traction slip according to claim 16 wherein said correction factor is developed by stepwise adaptation.
 18. A method for controlling traction slip according to claim 16 wherein said correction factor is developed by reduction of said differences with said correction factor included.
 19. A method for controlling traction slip according to claim 16 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor reduced by a specific amount at predetermined time intervals if Δ is zero.
 20. A method for controlling traction slip according to claim 19 wherein said correction factor is reduced by identical amounts if the predetermined time intervals are identical.
 21. A method for controlling traction slip according to claim 20 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 22. A method for controlling traction slip according to claim 16 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER -v.sub.a)-f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor reduced by a specific amount at predetermined time intervals if Δ is greater than zero.
 23. A method for controlling traction slip according to claim 22 wherein said correction factor is reduced by identical amounts if the predetermined time intervals are identical.
 24. A method for controlling traction slip according to claim 23 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 25. A method for controlling traction slip according to claim 16 wherein said correction factor is developed according to the following:

    Δ=(K×v.sub.ER ×v.sub.a)-f(v.sub.na1, v.sub.na2)

where: v_(ER) is the rotational speed of a driven wheel having a smaller than normal diameter v_(a) is the rotational speed of the second driven wheel v_(na1) is the rotational speed on one non-driven wheel v_(na2) is the rotational speed of the second non-driven wheelwith said correction factor raised by a specific amount at predetermined time intervals if Δ is greater than zero.
 26. A method for controlling traction slip according to claim 25 wherein said correction factor is raised by identical amounts if the predetermined time intervals are identical.
 27. A method for controlling traction slip according to claim 26 wherein said time interval is within the range of 10 to 100 msec and said correction factor is limited to a range of 0.8 to 1.00.
 28. For an automotive vehicle having an undriven axle with two wheels mounted thereon and a driven axle with two wheels mounted thereon, one of said wheels mounted on said driven axle being a spare wheel which is smaller in diameter than a normal wheel, a circuit configuration for controlling traction slip of said automotive vehicle comprising:means for developing speed rotation signals representative of the rotating speeds of said wheels; means for developing a correction factor signal prior to traction slip control; said means for developing said correction factor signal including:(a) first comparison means responsive to said speed rotation signals representative of said rotating speeds of said wheels mounted on said undriven axle for developing a first difference signal representative of the difference in said rotating speeds of said wheels mounted on said undriven axle, (b) second comparison means responsive to said speed rotation signals representative of said rotating speeds of said wheels mounted on said driven axle for developing a second difference signal representative of the difference in said rotating speeds of said wheels mounted on said driven axle, and (c) third comparison means responsive to said first and said second difference signals for developing a correction factor signal representative of the between said difference in said rotating speeds of said wheels mounted on said undriven axle and said difference in said rotating speeds of said wheels mounted on said driven axle; means for adapting said speed rotation signal representative of said rotating speed of said spare wheel by said correction factor signal to said speed rotation signal representative of said rotating speed of the other wheel mounted on said driven axle; means for establishing a slip threshold representative of a traction slip level which must be exceeded before traction slip control can commence; means for developing a vehicle speed signal representative of the speed of said automotive vehicle; means for comparing said vehicle speed signal and said speed rotation signals, including the speed rotation signal representative of the rotating speed of said spare wheel adapted by said correction factor signal, to develop a signal representative of the onset of traction slip; and means for comparing said signal representative of the onset of traction slip with said slip threshold to determine when the onset of traction slip exceeds said slip threshold. 